This invention relates generally to image reconstruction using data collected in a computed tomography scan and, more particularly, to generating an volumetric computed tomography imaging using half-projection data.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
In volumetric CT, data is collected using a large flat panel digital x-ray device, or detector, having a plurality of pixels arranged in rows and columns. More specifically, the imaging system includes an x-ray source and at least one solid-state x-ray detector. To generate volumetric images, at least one of the x-ray source and the x-ray detector are rotated around the object of interest. For each identified view, x-rays are emitted from the x-ray source toward the x-ray detector and projection data is collected for the view.
Because of magnification in cone beam geometry, a system having even the largest available flat panel detector has a useful scan field of view too small to cover a typical patient. To overcome limited panel size, the panel can be offset from iso-center to increase scan field of view by utilizing 2.pi. rotation of projections. In such a configuration, each projection is truncated and image artifacts typically result.